Field of the Invention
The invention of this application focuses on advances in photonic frequency combs. These find a wide variety of applications, including data harvesting and pattern recognition. An example of a system taking advantage of optical signal processing using stabilized optical frequency combs can be found in U.S. Pat. No. 7,917,039, incorporated-herein-by reference. Related improvements in optical networks are also set forth in U.S. Pat. No. 7,848,655. This invention focuses on improved signal processing systems, employing an improved frequency modulator.
Background of the Invention
The proliferation of applications for photonic frequency combs has fueled the search for sources which can deliver large optical frequency component spacing, narrow optical linewidth, and excellent RF phase noise and stability with small footprints, electrical efficiency, and ease of use. For example, applications such as optical arbitrary waveform generation and coherent communication require access to individual comb components necessitating comb spacing in the multi-gigahertz region [1]. The burgeoning field of multi-heterodyne spectroscopy utilizes a narrow linewidth frequency comb to probe the phase and amplitude information of an unknown signal with different periodicity [2]. This field can also benefit from easily tunable frequency comb spacing by granting control over the detuning frequency between the two signal pulse-trains.
Continuous wave (CW) optical injection locking of semiconductor-based harmonically mode-locked lasers has been shown to produce tunable, gigahertz-spaced frequency comb outputs with high optical and RF signal to noise ratio (SNR), and reduced phase and amplitude noise [3]. By suppressing all but one optical axial mode group via gain competition, an optical frequency comb is generated at the repetition frequency, which is then easily tunable at steps equal to the fundamental cavity frequency, typically tens of MHz. Stand-alone sources are also possible using injection locking in a Coupled Opto-Electronic Oscillator (COEO).
Such injection locking schemes necessitate active stabilization of the laser fiber cavity relative to the injection frequency. Previously, a modified Pound-Drever-Hall (PDH) scheme has been used which requires phase modulation (PM) of the injection signal before injection [3]. An inherent effect of this is that the PM sidebands are injected into the cavity and are then modulated at the repetition rate, producing unwanted carrier sidebands in RF and optical spectra.
As those of skill in the art will recognize, improvements in optical signal processing systems also benefit from improvements of the elements of those systems. Intensity modulators are one of the key components in signal processing, optical communication and photonic analog to digital converters (ADC). Recently, a linear intensity modulator based on an injection locked resonant cavity with gain has been shown and experimental results for a CW input light have been demonstrated. It is well known that an injection locked resonant cavity with gain serves as an arcsine phase modulator [6]. When the arcsine phase modulated light, which is a function of frequency detuning between the cavity and the injection seed, combines with reference arm, it produces an intensity modulated CW light directly proportional to modulating signal [7].
The linearity of the response of this modulator and the possibility of gain at the output are inherent in the above design and no linearization technique is used. This technique has shown 95 dB signal to noise ratio and 120 dB Hz2/3 spur-free dynamic range using a VCSEL as the resonant cavity, however, it is limited to using CW light. To adapt this technique for input light signals that are pulsed and periodic, one must use a resonant cavity with multiple resonances within the gain bandwidth.